1. Field of the Invention:
This invention relates to apparatus for scrambling information signal, such as television signal, and more particularly, to apparatus for scrambling the television signal in accordance with specified code signal and changes in the scene of the picture of the television signal.
2. Description of the Prior Art:
A scrambling broadcasting system is conventionally known in which a television signal of a specific program is scrambled and transmitted from a broadcasting station and a subscriber who contracts with the broadcasting station and has a decoder can view the specific program. In such a system, a broadcasting station transmits a scrambled information signal and a key signal for decoding it. A scrambled information signal is obtained by inverting a polarity of a television signal or reversing the signal order at a predetermined period. A non-subscriber is not therefore able to view a correct image or hear sound even if he can receive the scrambled information signal. Accordingly, only subscribers who have contracts with the broadcasting station can unscramble the scrambled information signal in accordance with the key signal and can view and hear the correct image and sound.
In a conventional scrambling broadcasting system as described above, when a scrambled information signal is received by a non-subscriber, the received picture must be non-sensible. For this purpose, various scrambling methods have been proposed.
One scrambling method is known in which a television signal is inverted at a predetermined field period. According to this method, when a scrambled signal is subject to distortion influence by a transmission system, unscrambling at a receiving side may not result in reproduction of an original waveform. For example, when a polarity-inverted signal is reinverted by a decoder, a level difference may be generated or the waveform may be distorted. When an unscrambled signal is supplied to a TV receiver, flicker occurs at the polarity-inverted portion of the picture. Such flicker is particularly noticeable in the case of a still image. When a scrambled program broadcast from a station is recorded by a VTR, flicker tends to be caused more frequently since a VTR is nonlinear in a DC manner as well as in an AC manner.
In order to overcome the above-described disadvantage, U.S. Pat. No. 4,340,906 discloses improved scrambling system, in which the polarity of the television signal is inverted or reversed when the scene change of the picture in the television signal. According to the system, the flickers in the picture will not be noticeable to the viewers. However, the system has a defect that a scrambling code is easily unscrambled even by viewers who do not contract with the broadcasting station, because the scramble code is determined only by the content of the picture in the television signal.